Amplifier with controllable band



Aug. 13, 1935.

WALTER VAN B. ROBERTS IISMW I@ f ./JUM/ Reissued Aug. 13, 1935 UNITEDSTATES PATENT OFFICE AMPLIFIER WITH CONTROLLABLE BAND PASS WIDTH ofDelaware Griginal No. 1,907,669, dated May 9, 1933, Serial No. 350.950,March 29, 1929.

Application for reissue September 28, 1934, Serial No. 746,025

20 Claims.

My present invention relates to amplifiers, and more particularly, to anamplifier with a controllable band pass width.

It is extremely desirable in the construction and operation of modernbroadcast receiving sets, that they should pass as uniformly as possiblea band of frequencies of 10 kilocycles in width, in order to preservefidelity of signal reproduction. However, when distant or weak signalsare being received, it has been found that, it is often moreadvantageous to sacrifice some fidelity, by decreasing the band offrequencies passed, inasmuch as this expedient eliminates considerableinterference.

Now, I have discovered a method for making a broadcast receiving circuitsharply selective and very sensitive at the same time, or less sensitivebut having band pass characteristics most favorable to fidelity, themethod including the automatic reduction of the amplification inproportion to the increase of fidelity, as great fidelity is usuallyuseful only on fairly strong signals.

Accordingly, it is one of the main objects of my present invention toprovide an amplifier to be used in connection with tuned radio frequencyreceivers or intermediate frequency amplication in superheterodynecircuits, which amplifier includes means for adjusting the fidelity ofsignal reproduction, and means for automatically adjusting thesensitivity of the amplifier in accordance with the width of the band offrequencies passed.

Another important object of the invention is to provide a coupling meansbetween one or more stages of amplification in a receiving circuit,which coupling means includes an essential and an auxiliary circuit, thecoupling between the latter two circuits being adjustable whereby theselecting characteristic of each stage is broadened and the amount ofamplification automatically reduced.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims, the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically one circuit organization whereby my invention may becarried into eifect.

In the drawing,

Figure 1 shows diagrammatically a superheterodyne circuit embodying myinvention,

Figure 2 is a graphic representation of the (Cl. Z50- 20) operation ofthe amplifier in Figure l, Without my invention,

Figure 3 is a graphic representation of the operation of the amplifierin Figure l, including my invention, 5

Figure 4 shows graphically the operation of the amplifier of the circuitof Figure l to accentuate the high frequency components of the band.

Referring to the circuit shown in Figure l, 10 the grounded antennacircuit A, G is coupled as at M, to the usual radio frequency amplifier,the amplified output of the latter being impressed upon the input of afrequency changing element, it being understood that the latter elementhas 15 impressed upon it the output from a local oscillator.

rlhe intermediate frequency from the frequency changer is amplified inan intermediate frequency amplier, the latter comprising one or morestages 20 of amplification as desired, the amplified output therefrombeing detected and utilized in any well known manner, which may bephones, a loud speaker, and the like.

My invention includes an adjustment inserted 25 in the coupling meansbetween successive tubes in the intermediate frequency amplifier anddetector unit, it being understood that the hereinafter disclosure,while specifically discussed with reference to intermediate frequencyampliers of su- 30 perheterodynes, is also operative in case of tunedradio frequency receivers. The intermediate frequency amplifier, shownin Figure 1, includes a tube I, of the well known screen-grid type, theinput circuit of which has impressed upon it the output from thefrequency changer, the screen element of said tube being biased by apotential source S.

The plate circuit of the tube is energized by a. source B, the output ofthe tube being impressed lll upon the input circuit of a secondscreen-grid tube 2, the screen element of which is also biased by asource S".

The grid elements of the tubes I and 2 are biased by a source C, theinput circuit of the tube 2 including the secondary coil 4 of theecupling M1, coil E, and a capacity C1. A resonant circuit, hereinaftercalled the auxiliary circuit" comprising a coil 5 and a capacity C2, isvariably coupled to the input circuit by virtue of variable 60 mutualinductance M2 between coils 5 and 6.

It should be noted that coil 5 is preferably not coupled to coil 4. Solong as this is avoided, coils 4 and 6 may be physically the same coil,coils 3 and 5 having no mutual inductance with each 65 other, but bothbeing coupled to the input circuit. The plate circuit of the tube 2 isenergized by a source the amplied output of the tube 2 being detectedand utilized as described heretorcre I find it satisfactory to utilize aB source of et volts, and source of d5 volts.

The input circuit of tube 2 and auxiliary circuit are both tuned to thefrequency which is to be amplified. If the coupling of the auxiliarycirruit to the input circuit is made zero the amplification of theampliner system will be a simple resonance curve as shown in Figure 2,and several stages of amplification will consequently result in an elynarrow band. for example of Width P1. it being, additionally, noted thatby proper choice of Mi the amplification with such intcrstage couplersmay be made the maximum of which the tubes i and 2 are capable. InFigure 2 I have assigned an arbitrary value A to the amplificationsecured when my adjustment is not employed.

On the other hand, if M2 is increased from zero by the proper amount,the selecting characteristic of the stage is broadened, and renderedapproximutely fiat-topped, as shown in Figure 3, and the amount ofamplification secured is also reduced. The value Ai, in Figure 3, showsthe amount of amplification secured with the auxiliary circuit ti. Czadjustably coupled by the proper amount to the input circuit of tube 2.

It will also be noted that the width of the band frequencies now passedwith approximate unformity has been increased to a value P2 which isconsiderably broader than the value P1. Inasmuch as goed fidelity isonly useful on fairly strong signals, it is a distinct advantage toreduce the amplification in proportion as the fidelity is increased, andas shown in Figure 3 my invention automatically provides thiscompensation.

If for any reason there is a lack of high audio frequency response inthe loud speaker (this might be caused, for example, by the use of gridleak and condenser detection or by the selecting systems in the radiofrequency selector) such lack of high audio frequency output may becompcnsated by increasing the coupling M2 shown in Figure l until theselecting characteristic is as shown in Figure 4, which depicts theaccentuation of the high frequency components of the band of frequenciespassed.

In Figure 4 the width of the band passed has been increased to P3, thusincluding and accentuating the high frequencies desired.

My invention differs basically from the use of loosely coupled tunedcircuits such as often ernployed in superheterodyne circuits, in thatwith the present system the ability to reject interference of aconsiderably diiferent frequency from the desired signal frequency, isnot greater than the ability of a single tuned circuit. Thisdisadvantage, however, is not grave, because several stages ofamplification, each with a single tuned circuit, are sufficientlyselective, especially when the selection provided by the radio frequencycircuits is considered.

It may also be pointed out. that an additional advantage of the presentadjustment, is the increased ampliflcation secured as the selectivity isincreased by loosening the coupling. It will be observed that this iscontrary to the operation in the type of superheterodyne circuitmentioned above, where a loosening of the coupling between circuitsresults in a reduction of amplification accompanying increasingselectivity.

As stated heretofore, a plurality of stages of ampiiiication may be usedin the intermediate frequency amplifier, shown in Figure l, and auni-control device can be employed to vary the coupling-rs between theinput or essential and auxiliary circuits simultaneously.

It will thus be seen that I have devised :i method of, and means for,making a receiving circuit sharply selective and very sensitive at thesaine tin..c, or sensitive but having band pass characteristics moetfavorable to fidelity, the means involving an adjustment associatedwitl'i the couplings between the tuned stava-1 of the amplifier andauxiliary tuned circuits.

While I have indicated and described only one system for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the par-- ticularorganization shown and described, but that many modifications in thecircuit arrangement, as woll as in the apparatus employed, may be madewithout departing from the scope of my invention as set forth in theappended claims.

What I claim is:

An amplifier comprising one or more stages of amplification free of anytendency to oscillate, coupling means between each stage. each couplingmeans including essential and an auxiliary circuit, the coupling betweenthe latter two circuits being adjustable and so adjusted that thesensitivity of each stage is automatically reduced when the selectivityis decreased.

2. An amplifier comprising a plurality of stages of amplification freeof any tendency to oscillate. coupling means between each stage. eachcoupling including moans acljustably associated therewith forautomatically reducing the sensitivity of a stage when the selectivityof the latter is decreased.

3. A coupling for a stage of amplification free of any tendency tooscillate compris'ng an essential circuit in the input of said stage.and an auxiliary circuit adjustably coupled to the essentlal circuit andso adjusted relative thereto that the sensitivity of the stage isautomatically reduced when the selectivity of the latter is decreased.

4. The method of controlling the selectivity of a high frequencyamplifier including at least two stages of amplification in cascade.which consists in coupling a resonant circuit to the coupling betweensaid stages, and varying the coupling between said resonant circuit andd interstaee coupling; to secure an inverse relation between thefrequency band width transmitted through the amplifier and theamplification of the band.

5. The method of controlling the selectivity of an intermediatefrequency amplifier including at. least two stages of amplification incascade, which consists in coupling a resonant circuit to the couplingbetween said stages, and varying the coupling between said resonantcircuit and said interstage coupling while maintaining said interstagecoupling xedly tuned to said intermediate frequency to secure an inverserelation between the intermediate frequency band width transmittedthrough the amplifier and the amplification of the band.

6. A method of operating a receiver including a high frequency amplifierprovided with an input circuit resonant to the frequencies to beamplified, which consists in selectively varying the characteristics ofsaid input circuit to secure an inverse relation between the frequencyband width transmitted through the amplifier and the amplification ofthe band.

7. A method of operating a superheterodyne receiver including anintermediate frequency amplifier provided with an input circuit resonantto the frequencies to be amplified, which consists in selectivelyvarying the characteristics of said input circuit to secure an inverserelation between the frequency band width transmitted through theamplifier and the amplification of the band while maintaining said inputcircuit resonant to said frequencies at all times.

8. A superheterodyne receiver including an intermediate frequencyamplifier network between the first and second detectors, said networkcomprising a tube having a tuned input circuit resonant to the operatingintermediate frequency, said circuit including at least one coil and acondenser, a tube preceding said circuit and including a coil in itsplate circuit magnetically coupled to said tuned circuit, a resonantabsorption circuit coupled to said tuned circuit, said absorptioncircuit being tuned to said intermediate frequency, said absorptioncircuit comprising at least a coil and condenser, said last coil beingmagnetically coupled to said tuned circuit coil, and means forregulating the degree of selectivity of said receiver comprising adevice for adjusting the reaction of said absorption circuit on saidtuned circuit.

9. A superheterodyne receiver including a selective coupling network fortransmitting intermediate frequency currents from one tube to the next,an absorption circuit tuned to the intermediate frequency andelectrically associated with said coupling network, and means forvarying the reaction of said absorption circuit upon the couplingnetwork.

l0. In a receiver as defined in claim 9, said absorption circuitcomprising at least a coil and condenser, and said varying means beingoperatively associated with at least said absorption circuit.

1l. In a receiver as defined in claim 9, said coupling network includingat least one tuned circuit magnetically coupled to the absorptioncircuit.

l2. A superheterodyne receiver including a selective coupling system fortransforming intermediate frequency currents from one tube to the next,an absorption circuit tuned to the intermediate frequency and associatedwith said selec tive system, and means for varying the reaction of saidabsorption system upon said selective system for the purpose of Varyingthe width of the band of intermediate frequencies passed by saidselective system.

13. In a receiver of the superheterodyne type, and in combination withthe intermediate frequency transmission network thereof, a device foradjusting the degree of selectivity of the receiver, said devicecomprising a signal energy absorption network reactively coupled to thetransmission network, and means for varying at will the absorptioneffect of said absorption network.

14. In a receiver of the superheterodyne type, and in combination withthe intermediate frequency transmission network thereof, a device foradjusting the degree of selectivity of the receiver, said devicecomprising a signal energy absorption network tuned to the frequency ofsaid transmission network reactively coupled to the transmissionnetwork, and means for varying at will the absorption effect of saidabsorption network.

l5. In a receiver of the superheterodyne type, and in combination withthe intermediate frequency transmission network thereof, a device foradjusting the degree cf selectivity of the receiver, said devicecomprising a signal energy absorption network reactively coupled to thetransmission network, and means independent of the receiver stationselector for varying at will the absorption effect of said absorptionnetwork.

.1.5. In combination with the intermediate frequency network of asuperheterodyne receiver, a device for regulating at will the degree ofselectivity of the receiver, said device comprising a resonant circuittuned to the intermediate frequency and coupled to said network, andmeans for adjusting the absorption of intermediate frequency energy fromsaid network by said resonant circuit.

17. In a device of the type defined in claim 16, said adjusting meansbeing chosen to vary said absorption from substantially zero to amagnitude sufficient to impart a broad selectivity characteristic tosaid network.

18. In a device of the type defined in claim 16, said resonant circuitbeing reactively coupled to the network, and said absorption adjustingmeans being capable of adjustment without effect on the tuning of saidnetwork and said resonant circuit.

19. The method of operating a receiver of the superheterodyne typeincluding an intermediate frequency amplifier provided with an inputcircuit resonant to the intermediate frequency employed, which includesselectively absorbing intermediate frequency energy from the said inputcircuit, and varying the said absorption to adjust the degree of'selectivity of said input circuit.

20. In a method as defined in claim 19, increasing the absorption ofsaid intermediate frequency energy to render the input circuit broadlyresonant, and decreasing the absorption to sharpen the tuning of theinput circuit.

WALTER VAN B. ROBERTS.

